Atomic force microscopes (AFM's) were developed, among other reasons, to meet a demand for accurately measuring critical dimensions (CDs) during integrated circuit (IC) fabrication. Critical dimensions constitute the width of a line or space, such as the width of a patterned line, the distance between two lines or devices, or the size of a contact, on a substrate identified as crucial for proper operation of the device being fabricated. Critical dimensions are measured on some or all substrates following any photolithography or etching process in which the dimensions are critical. Due to their high spatial resolution, AFM's are well suited for measuring critical dimensions during IC fabrication.
Generally, AFM's are equipped to sense atoms on or in sample surfaces, thereby providing atomic level surface imaging. AFM images are reconstructed from digital data acquired during grid scanning of a probe tip across the sample. Any AFM image represents the sample surface convoluted with the shape of the probe tip used to acquire the digital data. Moreover, the shape of the probe tip may change during scanning due to wear or the probe tip may become contaminated by foreign debris, thus introducing additional distortions into the image data. Measurement accuracy is improved if the distorted image data is corrected using a correction factor developed from the dimensions of the probe tip. The correction effectively deconvolves the shape of the probe tip from the digital data so that the AFM image accurately reflects the actual structure of the sample surface.
The shape of the probe tip may be determined by making physical measurements of the tip using a scanning electron microscope or deduced by reliance upon a known calibration standard or characterizer. As the dimensions of probe tips shrink for imaging shrinking IC features, the feature sizes approach the microscope resolution limit. For example, ten to thirty nanometer probe tips demand a resolution of one to two nanometers for accurate shape characterization. As a result, small probe tips are impossible to directly image with sufficient accuracy to provide correction factors for AFM image data. Hence, characterizers are required for deducing the shape of small probe tips, as disclosed in U.S. Pat. No. 6,810,354.
One class of conventional characterizers is structured as multiple trapezoidal pillars projecting from a surface with a spacing between adjacent pillars of approximately a few microns. A free end of each pillar is surrounded by an edge in the form of a thin outwardly-projecting lip that overhangs a trench separating adjacent pillars. As a result, the entrance to each trench is characterized by a pair of opposed edges. Each edge overhangs the trench by an overhang distance that is considerably less than one-third of the width between adjacent edges.
Such conventional characterizers are plagued by numerous deficiencies. One problem is that the pillar must be narrow to provide the best resolution for tip width measurements (which are made by scanning the probe tip over the pillar) and, concurrently, the overhang distance of the edge should be wider than the boot width of the probe tip. Satisfying both requirements would require an extremely thin pillar susceptible to fracture when struck by the probe tip during characterization. Another problem is observed when the probe tip and the characterizer pillar are aligned with a nonparallel relationship. This problem arises if, for example, the characterizer is mismounted in the AFM, the probe tip is mismounted in the AFM cantilever holder, or the characterizer pillars fail to meet specification and results in an inability to accurately characterize the boot shape. Specifically, in these situations, the probe tip may contact the sidewall of the pillar rather than the edge, which prevents accurate measurement of the boot shape on one side of the probe tip.
Yet another problem with conventional characterizers may occur if the AFM has a clamping function that prevents profiling below a given depth of a sample, typically a depth exceeding the length of the probe tip. The clamping function prevents false readings that may result from contact between the post of the probe tip and trench sidewalls, which is misinterpreted to be the probe tip contacting the base of a trench. The clamping function may interfere with the calibration process using conventional characterizers.
Conventional characterizers are limited in the extent to which the shape of a probe tip may be characterized. Conventional characterizers lack the ability to accurately characterize concavity on the bottom surface of the boot if the probe tip width is equal to, or less than, the width of the edges. Another limitation observed with conventional characterizers is that most only include parallel structures, which means that shape measurements may only be made in one direction as the probe tip is translated across the characterizer. As a result, the characterizer must be reoriented for characterizing additional portions of the probe tip, which cannot be dismounted from the AFM during characterization.
What is needed, therefore, is a characterizer for accurately determining the shape of an AFM probe tip that overcomes these and other deficiencies of conventional characterizers.